Vent valves as described by Parkinson in U.S. Pat. No. 3,280,838 are used on liquid transporting tanks to limit the gas pressure in the tanks by permitting gas to escape from and enter the closed tank. The vent valves also function to prevent leakage of liquid from the tank on surging of the liquid in the tank and limit the flow of liquid from the tank in the event the tank is partially or totally overturned. An example of a transport tank is shown by Brown et al in U.S. Pat. No. 2,669,250. When tanks of this type are filled with liquid, as gasoline, the gases and vapors of the tank are vented to the atmosphere. These vapors are a source of air pollution Many localities do not allow venting of tanks to the atmosphere during the filling operation. Special venting systems have been developed to avoid venting the vapors into the atmosphere. In one system, the tank is filled through a tight connection and the air and vapors are vented through a special valving structure and suitable lines that carry the vapors and gases to a confined area separated from the atmosphere. After the tank is filled, the special valve is closed and not used during transportation of the tank.